1. Field of the Invention
This invention relates to an electromagnetic contact device, and more particularly, it is concerned with an electromagnetic contact device to perform on-off control operations of an electric path, through which electric power is supplied to a device such as an electric motor.
2. Description of the Prior Art
There has previously been used a device as shown in FIG. 1 of the accompanying drawing as an electromagnetic contact device.
That is, as shown in the drawing, a box-shaped fitting plate 32 is provided for mounting the main body of the electromagnetic contact device on a mounting table. The main body of the electromagnetic contact device is screw-fitted on the mounting table through a plurality of fitting holes 33 formed in the fitting plate 32.
A base 11 made of an insulating material is fixedly secured on this fitting plate 32 by means of fitting screws 28. A terminal plate 12 to connect the electromagnetic contact device with a main circuit is, in turn, fixed on this case 11. A fixed contact piece 13 provided with a fixed contact point 14 is secured on this terminal plate 12 in an electrically conductive manner with the same. Further, as shown in FIG. 1, a cross-bar 4 made of an insulating material is provided on this base 11 in a manner so as to be movable up and down. The cross-bar 4 is upwardly biased in FIG. 1 by a take-off spring 31 compressed between a spring receptacle 100 formed at the bottom end of this cross-bar 4 and the fitting plate 11. In a supporting hole 4a formed in the cross-bar 4, there is inserted a movable contact piece 8 provided with a movable contact 6 disposed in confrontation with the above-mentioned fixed contact 14. The movable contact piece 8 is downwardly biased, in FIG. 1, by a spring 9 for the contact piece and installed in a compressed state between a spring receptacle 101 formed in the cross-bar 4 and a spring support 102.
In order to drive the above-mentioned movable contact 6 and to perform the contact opening and closing actions in cooperation with the fixed contact 14, there is provided a drive mechanism to be explained in the following.
A fixed iron core 20 manufactured by laminating silicon steel plates is disposed on the fitting plate 32. A plurality of pins 24 are inserted into this fixed iron core 20. Both ends of each of the pins 24 are capped buffer rubber 25. A buffer spring 103 is provided between the fitting plate 32 and the buffer rubber 25. A movable iron core 18 is fixedly fixed at the bottom end of the above-mentioned cross-bar 4 by a pin 19. The movable iron core 18 is disposed in confrontation with the fixed iron core 20 with a predetermined space interval between them. An operating coil 22 is wound around a coil supporting frame 104 mounted on the fixed iron core 20 for attracting the movable iron core 18 to the fixed iron core 20 by imparting electromagnetic force to the latter. The operating coil 22 and a coil terminal 36 are mutually connected through a lead wire 43.
Furthermore, in order to extinguish an arc which occurs at the time of opening and closing the contact, an arc box 1 made of a heat-resistant material is fixed on the base 11 with screws 105. In this arc box 1, there are provdied grids 2 made of a magnetic metal material and so formed as to encompass the movable contact 6 and the fixed contact 14. By these grids 2, the arc is led out and extinguished.
An energizing voltage for the above-mentioned operating coil 22 is not required to be maintained at a constantly high level, but only a low level energizing voltage may be supplied to it after closure of the main circuit. Therefore, the energization of the operating coil 22 is accomplished by a circuit as shown in FIG. 2 of the accompanying drawing.
For rectifying an alternating current voltage from an AC power source 106 and supplying the energizing current to the operating coil 22, there is provided a rectifying circuit 107, to which a holding circuit 109 having an energizing voltage dividing resistor 108 for the operating coil 22 is connected in series. Moreover, a closure completion switch 110 is connected in parallel with the holding circuit 109. This closure completion switch 110 is so constructed that it may perform off-operation at the closure timing of both fixed and movable contacts 14 and 6 when the main circuit is closed, and that it may perform on-operation at the opening timing of contacts 14 and 6 when a voltage to be applied to the above-mentioned operating coil 22 becomes lowered. Furthermore, for performing the energizing operation of the operating coil 22, a switch 111 is connected to the AC power source 106.
In the following, explanations will be given as to the functions of the conventional electromagnetic contact device of the above-described construction.
When the switch 111 is closed in a state of contacts 14, 6 being separated, an alternating current voltage from the AC power source 106 is rectified by the rectifying circuit 107 on account of the closure completion switch 110 performing its on-operation, whereby an energizing current of a large capacity is supplied to the operating coil 22. As a result of this, electromagnetic attractive force occurs between the fixed iron core 20 and the movable iron core 18, whereby the movable iron core 18 is attached to the fixed iron core 20 against the force of the take-off spring 31. With this attraction, the cross-bar 4 connected with the movable iron core 18 shifts toward the fixed iron core 20, and the movable contact 6 of the movable contact piece 8 supported by the cross-bar 4 contacts the fixed contact 14 of the fixed contact piece 13. In this case, since a core gap formed between the movable iron core 18 and the fixed iron core 20 is larger than the contact gap between the movable contact 6 and the fixed contact 14, the cross-bar 4 further shifts toward the fixed iron core 20 to a point past the contact position of the above-mentioned contact points. On account of this, the spring 9 for the contact piece is deformed by compression, and the spring pressure is transmitted to the movable contact piece 8 through the spring support 102 to close the contact points with a predetermined contact pressure.
In the state of the contacts 14, 6 being closed, the closure completion switch 110 is switched to perform its off-operation, so that the alternating current voltage from the AC power source 106 is rectified by the rectifying circuit 107 only after being divided by the holding circuit 109, so that only a small energizing current is supplied to the operating coil 22.
When the drive voltage which has been applied to the operating coil 22 is removed, the electromagnetic attractive force between the fixed iron core 20 and the movable iron core 18 is extinguished, and the cross-bar 4 shifts away from the fixed iron core by the biasing force of the compressed take-off spring 31, whereby the contact points are separated. At this instant, there is generated an arc across the movable contact point 16 and the fixed contact point 14. However, this arc is drawn into the grids 2 surrounding the portion of the above-mentioned contact points, cooled and cut apart for extinction.
However, in view of the fact that the conventional electromagnetic contact device is so constructed that it may perform its off-operation at the closure timing of contact points 14, 6 when the closure completion switch 110 closes the main circuit, and that it may perform on-operation at the opening timing of contact points 14, 6 when a voltage to be applied to the operating coil 22 becomes lowered, as mentioned in the foregoing, there arise various problems to be described hereinbelow due to even a slight error in the on-and-off operation timing of the closure completion switch 110.
FIGS. 3A and 3B are graphical representations indicating a relationship between a spacing of iron cores 20, 18 and an iron core attracting force at the closure of the main circuit, and at the time of lowering of the voltage applied to the operating coil 22, respectively. In the drawing, the biasing force of the take-off spring 31 is indicated by a dash line, the biasing force of the contact piece spring 9 by a dot-and-dash line, and the core attracting force by a solid line.
In the state of the main circuit being open, when the switch 111 is closed, the core attracting force gradually increases as shown in FIG. 3A, during which the distance between iron cores 20, 18 decreases gradually from a point H.sub.1. In this case, if it is assumed that the off-operation timing of the closure completion switch 110 comes before the closure of contact points 14, 6, then the closure completion switch 110 may perform its off-operation as soon as the distance between cores 20, 18 reaches a point H.sub.2 where the core attracting force decreases from F.sub.1 to F.sub.2 and again increases. However, at a time instant when the distance between iron cores 20, 18 reaches a point H.sub.3, i.e., when contact points 14, 6 are closed, since the core attracting force F.sub.3 is weaker than a sum F.sub.4 of the biasing force of the take-off spring 31 and the biasing force of the contact piece spring 9, the movable iron core 18 stops its shifting at a position where the distance between cores 20, 18 reaches the point H.sub.3. While maintaining the distance between iron cores 20, 18 at the point H.sub.3, the core attracting force gradually increases and, at a point where the core attracting force becomes stronger than the sum F.sub.4 of the biasing force of the take-off spring 31 and the biasing force of the contact piece spring 9, the movable iron core begins to shift again, whereby iron cores 20, 18 come into contact.
As the consequence of this, no predetermined contact pressure can be obtained between contact points 14, 6 due to stoppage of the movable iron core 18, as mentioned above, to disadvantageously result in fusion of contact points 14, 6 or burning of the operating coil 22.
When a voltage to be applied to the operating coil 22 lowers when the main circuit is closed, the core attracting force gradually decreases from a point F.sub.5 as shown in FIG. 3B, during which the distance between iron cores 20, 18 increases gradually. In this case, if it is assumed that the on-operation timing of the closure completion switch 110 comes before separation of contact points 14, 6, the closure completion switch 110 performs its on-operation when the distance between iron cores 20, 18 reaches a point H.sub.4, whereby the core attracting force increases from F.sub.6 to F.sub.7, and decreases again. However, at a time instant when the distance between iron cores 20, 18 reaches the point H.sub.3, i.e., when contact points 14, 6 move apart, since the iron core attracting force F.sub.8 is stronger than the biasing force of the take-off spring 31, the movable iron core 18 stops its movement at a position where the distance between iron cores 20, 18 reaches the point H.sub.3, and the iron core attracting force gradually decreases while maintaining the distance between iron cores 20, 18 at H.sub.3. Then, as soon as the core attracting force becomes weaker than the biasing force of the take-off spring 31 at F.sub.9, iron cores 20, 18 are separated.
As the result, no predetermined contact pressure can be obtained between contact points 14, 6 due to stoppage of the above-mentioned movable iron core 18, and the contacts 14, 6 are fusion-bonded together or the operating coil 22 is burnt. In particular, when the voltage to be applied to the operating coil 22 lowers to a level between F.sub.6 and F.sub.9, owing to the troubles in the device, the above-mentioned fusion-bonding between the two contact points 14, 6 and burning of the operating coil 22 becomes considerable, because the above-mentioned movable iron core 18 continues its stoppage.